Josephson devices are known in the art and have primary advantage in that they display low power dissipation (about 1 microwatt) and fast switching speeds (approximately tens of picoseconds). These are attributes for use as elements in super fast computers and they have been suggested for this application. The progress in the superconductive technology, and in particular the development of Josephson devices for memory and logic, has been quite substantial in the past decade.
Even prior to the discovery of the Josephson effect, it was pointed out that certain basic problems in large scale integration and in extreme miniaturization of electronic circuits may require new types of device structures. For example, although device dimensions decrease with decreasing line width, the number of connections per device remain more or less the same. Distributed device structures that avoid attenuation and dispersion of signals, and novel system concepts have been suggested in the art as alternatives. Further, the concept of distributed devices can easily be extended to the Josephson technology; however, only limited progress has been made so far as is apparent from K. Nakajima et al, J. Appl. Phys. 47, 1620 (1976); T. A. Fulton et al, Appl. Phys. Lett. 22, 232 (1973); and T. A. Fulton et al, Proc. IEEE 61, 28 (1973).
The present invention describes a distributed Josephson logical device based on the principle of selective control of the movement of Josephson solitons. These are isolated fluxoids, of a type known in the art and described more completely in J. Rubenstein, J. of Math. Phys. 11, 258 (1970); A. C. Scott, Nuovo Cimento B69, 241 (1970); and T. A. Fulton et al, Solid State Comm. 12, 57 (1973); and T. V. Rajeevakumar et al, Phys. Rev. B27, 5432 (1980). It is known in the art that a Josephson transmission line (i.e., a long one dimensional Josephson junction) can support the propagation of Josephson solitons as reported by the aforementioned T. V. Rajeevakumar reference. The soliton can be generated in the Josephson transmission line by known techniques and can be made to propagate and accelerate through the Josephson transmission line under the influence of the Lorentz force due to a bias current in the Josephson line.
The concept of using a soliton, or fluxoid, to carry information is known in the art, as is the concept of moving these information-representing solitons along the Josephson transmission line. Further, it is known that a soliton brought to the intersection of Josephson transmission lines can be made to follow one or another path away from the intersection. This is described in the Nakajima reference described previously, where that concept is used for the design of logic networks. In that paper, the authors describe "turning points" of two types: in one type a single flux quantum propagating to the trigger turning point (TTP) on any one line will initiate a single flux quantum on all connected lines, and in the other type, a second turning point named the selective turning point (STP), a single flux quantum propagating toward the point on any one line will initiate a single flux quantum on only one connected line. The determination of which line a single flux quantum propagates in depends upon the bias current of each line, the local applied magnetic field, and the junction geometry. Basic logic circuits using these turning points are described in this reference.
The structure of Nakajima et al has many disadvantages and is not a practical circuit. For example, the mechanism for selection of the path to be followed by the single flux quantum depends upon a very delicate balance in the competition between the device internal damping (.GAMMA.), which in turn is highly dependent upon the choice of device materials, the bias levels (.gamma.) of the control signals, and the choice of boundary conditions at the intersections of the turning points. Because there are these competing forces, selection of a desired path for propagation of the soliton depends on factors other than just the presence or absence of the control signal. This means that the margins for path selection are very limited. Furthermore, any variations in device design across the chip or in the geometry of the various devices will lead to problems. Still further, the two types of turning points require different structure, and for this reason the logic chip will have to be fabricated from devices having different designs. This puts an additional constraint on the fabrication and on the number of masking steps which are required.
In the Nakajima et al circuits, undesired solitons will be generated at the turning points and these will not be easily removed. Also, effective isolation between the output of the devices and the input is not achieved when the control signals are directly coupled to the Josephson transmission lines. Additionally, in LSI circuits it is desirable to use the same control signal amplitudes throughout in order to improve circuit reliability and margins. However, in Nakajima et al, the device internal damping will vary throughout the chip and it will be virtually impossible to provide LSI circuits using that approach. Further, the need for a multitude of bias levels for control is very impractical when large scale integrated circuits are to be provided. Still further, the multi-level junctions required when fabricating the STP turning point leads to very difficult fabrication steps and processing yields will be low.
The switching device and circuits of the present invention solve these problems by using a different mechanism for soliton path selection. Rather than having path selection depend upon a delicate balance of competing forces, path selection in the present invention is controlled solely by a single bias level. That is, the direction taken by the soliton at an intersection of transmission lines depends only upon the presence or absence of a control signal of appropriate polarity. Additionally, reflections of the soliton at the intersection of the transmission lines are eliminated and unwanted solitons are destroyed at the intersection. Still further, the solution of the present invention provides isolation between input and output circuits and is therefore useful in the design of logic circuits.
Accordingly, it is primary object of the present invention to provide an improved technique for selecting the path along which a soliton will travel.
It is another object of the present invention to provide a technique for path selection of a soliton at the intersection of two or more transmission lines, where the path selection depends only upon the presence or absence of the control signal.
It is another object of the present invention to provide a path selection scheme for solitons in Josephson transmission lines wherein constant amplitude control signals are used to provide selection over the entire superconducting chip having a plurality of soliton steering devices thereon.
It is a further object of the present invention to provide a soliton deflection scheme for directing solitons to selected transmission lines wherein unwanted solitons are effectively dissipated.
It is a still further object of the present invention to provide a soliton steering device in which solitons do not experience unwanted reflections when approaching the intersection of two or more transmission lines along which the solitons could travel.
It is another object of the present invention to provide soliton steering devices for selection of paths along which solitons will travel which is easy to fabricate in a single level structure, and which has good margins for path selection.
It is another object of the present invention to provide a superconductive chip using Josephson solitons for memory and logic, wherein circuits having different functions can be fabricated from the same Josephson soliton path selection element.